The present invention relates to an apparatus for and a method of recording and reproducing holograms by recording a light interference intensity as a refractive index change when a plurality of light beams emitted from one light source are caused to interfere with each other in a recording medium, and reading the recorded information by detecting reproduced light that is generated when a portion (reference beam) of the plural light beams is applied to the recording medium.
FIG. 7 of the accompanying drawings is a schematic perspective view of a first example of a hologram recording and reproducing system.
The hologram recording and reproducing system shown in FIG. 7 is based on angle-multiplexing, and includes a laser beam source (not shown), a beam splitter 10, an LCD 11, a recording medium 12, a CCD 13, lenses 14, 15, and mirrors 16, 17.
For recording information in recording medium 12, a laser beam emitted from the laser beam source is divided by the beam splitter 10 into two beams, and one of the beams passes through the beam splitter 10 and is applied to the LCD 11. The laser beam is modulated into a bit pattern displayed by an SLM (spatial light modulator) of the LCD 11, and applied to the recording medium 12.
The other laser beam divided by the beam splitter 10, which is referred to as a reference beam, is reflected by the mirrors 16, 17 and applied laterally to the recording medium 12. In the recording medium 12, therefore, the two laser beams interfere with each other, and the interference intensity is recorded as a refractive index change.
In the angle-multiplexing system, the mirror 17 is rotated to change the angle of the interference beam to record different bit patterns on the SLM.
The hologram recording and reproducing system shown in FIG. 7 is disadvantageous in that the reference beam is applied at the same angle as the angle used for recording the bit patterns to reproduce the bit patterns on the CCD 13, and the recording medium is fixed in position, making it difficult to record and reproduce a large amount of information.
FIG. 8 of the accompanying drawings is a schematic perspective view of a second example of a hologram recording and reproducing system. hologram recording and reproducing system shown in FIG. 8 is based on shift-multiplexing, and includes a laser beam source (not shown), a beam splitter 20, an LCD 21, a recording medium 22, a CCD 23, lenses 24, 25, 26, and a mirror 27.
For recording information in recording medium 22, a laser beam emitted from the laser beam source is divided by the beam splitter 20 into two beams, and one of the beams passes through the beam splitter 20 and is applied to the LCD 21 by the reflection of the mirror 27. The laser beam is modulated into a bit pattern displayed by an SLM (spatial light modulator) of the LCD 21, and applied through the lens 24 to the recording medium 22.
The reference beam, which is the other laser beam divided by the beam splitter 20, is applied through the lens 25 laterally to the recording medium 22. In the recording medium 22, therefore, the two laser beams interfere with each other, and the interference intensity is recorded as a refractive index change.
The recording medium 22 is in the form of a disk, and is rotated by a rotating mechanism (not shown) to displace the recording position. In this system, a spherical wave produced by the lens 25 is used as the reference beam, and the recording medium 22 is rotated to record and reproduce different bit patterns for making it possible to record a large amount of information.
Though the hologram recording and reproducing system shown in FIG. 8 can record and reproduce different bit patterns at small spaced intervals in the rotational direction of the disk, without cross-talk, it is necessary to provide large spaced intervals for shifts in the radial direction of the disk in order to eliminate cross-talk, posing limitations on the recording density for information. This problem is addressed in “Shift multiplexing with spherical reference waves”, Barbastathis G., Levene M., Psaltis D., Appl. Opt., Vol. 35 (1996), p. 2403, for example.
In order to solve the problem of the above system, there has been proposed a system wherein a reference beam which passes through a diffuser, e.g., a frosted glass panel, to have its phase randomized is used to eliminate anisotropy for making it possible to record and reproduce different bit patterns at small spaced intervals for shifts in the rotational, radial, and transverse directions of the disk. This technique is disclosed in “Multilayer holographic data multiplexing with random encoded reference beam”, Markov, V. B., Millerd, J. E., Trolinger, J. D., SPIE Vol. 3864, p. 100, for example.
FIG. 9 of the accompanying drawings shows the manner in which a reference beam 32 which passes through a diffuser 31 to have it phase randomized is applied to a recording medium 33 in the above hologram recording and reproducing system.
The above document refers to the need for record and reproduce multiplexed data at spaced shift intervals greater than the speckle size of the reference beam so that the reproduced signal vs. noise ratio will meet the requirements of the above system which employs the diffuser. However, since it is not clear how much the spaced shift intervals should be greater than the speckle size of the reference beam for recording and reproducing multiplexed data, it is not easy to determine spaced shift intervals and multiplexed densities in actual system designs. Therefore, there is difficulty putting the proposed system into practical use.